Reflection of Multilayered Surfaces Measured

Michael J. Lander

Beetles’ rigid forewings, called elytra, often display multilayered structures and iridescent metallic colors. As one might imagine, their refractive index proves hard to measure accurately. To help address this challenge, Joseph A. Noyes and colleagues at the University of Exeter in the UK have developed a system that could allow more effective characterization and design of multilayered surfaces.

Scientists measured the refractive index of all the individual layers that make up Chrysochroa raja’s colorful forewings during illumination with white and laser light.

Prior to collection of the indices, the researchers revealed with a JEOL transmission electron microscope that the elytra of Chrysochroa raja comprise an average of 17 alternating light and dark contrasted layers ranging from 50 to 111 nm in thickness. These values gave the team a starting point for subsequent fitting of reflection data using mathematical models.

With either white light or three lasers — a Viasho 473-nm diode, a Coherent 543-nm HeNe and a 632-nm HeNe — the scientists illuminated elytral samples from four angles. Glan-Thomson prisms polarized each light type, and the laser beam passed through an optical modulator. An Ocean Optics fiber optic cable delivered reflected light to a spectrometer from the same company or, in the laser track, to a condensing lens and a Hamamatsu photodiode. Tinted filters restricted photodiode operation to between 0 and 1 V.

For the light and dark contrasted layers, the scientists obtained refractive index values of 1.68 and 1.55, respectively. To help calculate them, the researchers applied Fresnel equations to the reflection data of each layer to provide a model for the entire stack. The combination white and laser light data — a feature absent from most studies — ensured the absolute calibration of reflection spectra needed for effective modeling.

“We tried to avoid any sort of estimate and let the data tell us what the refractive indices were,” Noyes stated, emphasizing the technique’s suitability for many-layered systems. Previous groups, he noted, were unable to measure the individual layers’ indices of refraction; instead, they approximated the values based on the overall index. The system also overcomes other accuracy-limiting drawbacks of previous approaches, which include limited angles of incidence and the inability to account for polarization.

Noyes said that the researchers are examining another beetle that has more complex elytra. He explained that, if scientists determine the details of these and similar formations, they might use the information to synthesize materials with analogous structural and optical properties, such as high reflectivity.

Optics Express, April 2, 2007, pp. 4351-4358.